Process of producing vitreous silica



Patented May 10,1927.

UNITED. STATES PATENT OFF-ICE.

. v I m 7 LEVI B, KILLER, OF LYNN,- MASSAOHUSETTS, ASSIGNOB TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK.

PROCESS OF PROD UOING VITREOUS SILICA.

whereas for most of the industrial applica tions of vitreous or fused silica, a clear,

transparent product is desired. When clear,

transparent uartz crystal is fused in a 5 vacuum and t e fusion then is subjected to pressure, bubbles or cavities in the product are reduced to a negligible residue.

Clear quartz crystal, however,.is not distributed widely in nature. Although silica constitutes the. largest part of the earths crust and is found in an impure state almost anywhere, clear quartz crystal is sometimes. obtainable only with great difficulty. Quartz crystal which is milky in appearance v either wholly or in which clear and milky portions are intermingled, however, is abundant.

In accordance with my present invention a process is rovided whereby the milkiness or o acit'y o the vitreous product produced by ta fusion of milky uartz crystal can be materially reduced or e iminated. In accordance with my invention, the milky or partly milky quartz crystal is 5 heated to a temperature Just below fusion preferably in vacuo, the material then is cooled to a temperature at whichits structure becomes spon y or granulated and this product then is re eated in vacuo to fusion.

. 4 In carrying out my invention the milky.

uartz crystal is first freed from impurities, t on 1s heated in a container consisting of graphite or other suitable material to a temperature just below fusion which varies with different varieities of quartz. Generally fusion occurs at about 1700 C. A

' vacumn furnace such as shown in' Devers Patent 1,536,821 of May '5, 1925', may be used. The fusion temperature forany particular variety of quartz can be determined Application filed December 29, 1926. Serial No. 157,870.

by trial. The regular crystalline structure of the quartz is destroyed at about 1600 0., or less if theheat is kept on long enough. Byheating to such a temperature for a suflicient tlme the material assumes a new condition known as cristobalite which exists in two forms, alpha and beta, alpha being stable below about 270 C., and beta being stable above this. When this material is cooled'to a temperature below 270 C. it again becomes altered, becoming frosty in appearance somewhat like loaf sugar, having changed over into alpha cristobalite.

The conversion of beta to alpha cristobalite preferably, but not necessarily, is carried on in a vacuum. When reheated to about 270 C. it changes back to beta cristobalite, but the granulated a pearance remains. Tliecristobalite final y is fused in vacuo in a suitable furnace, such as disclosed, for example, in Devers Patent 1,536,- 821. The beneficial effect of the conversion of milky quartz crystal to alpha cristobalite preparato to fusion of the material in vacuo I be ieve can be ascribed to the 0 en-' ingof minute cavities as the crysta line form changes on going into'the alpha state and the escape of gas therefrom before the fusing point is reached. It is not necessary to convert the quartz entirely to cristol0 balite to secure good results. The vacuum-fused product may be subjected to any" of the fabricating procedures now in use I for the manufacture of various kinds of fused quartz roducts, for 86 exam lo, the lastic vacuumsed roduct may .extru ed into the form of tu 'sand rods by the use. of the apparatusjand the practice. of the process described in my prior Patents 71,549,597 of August 11, 1925, 00

and 1,562,115 of November 17, 1925.

WhatIclaim as new an desire to secure by Letters Patent of the United States, is

1. The step in the process of treating cloudy or milky, crystalline quartz before 96 conversion to the vitreous state which consists in heating said material to a temperature just below fusion and cooling sufliciently to produce a granular structure.

2. The process of converting milky quartz crystal to a clear, vitreous condition which perature adjacent to but below about 1700 consists in converting said crystal in vacuo (3., cooling the same to a temperature below to alpha-cristobalite and then fusing in a 270 0., and finally fusing said material in 1 vacuum. a vacuum.

a 3. The process of producing vitreous or In witness whereof, I have hereto set fused quartz from crystal quartz which my hand this 24th da of December, 1926. consists in heating said material to a tem- L VI B. MILLER. 

